(a) Field of the Invention
The present invention relates to a cellular wireless communication system of an orthogonal frequency division multiple access (OFDMA) method. More specifically, the present invention relates to an apparatus and method for erasure detection and soft-decision decoding in a cellular system receiver.
(b) Description of the Related Art
Conventionally, an orthogonal frequency division multiple access (OFDMA) cellular system, to make cell planning easy, uses a method of averaging inter-cell interference which assigns different frequency hopping patterns to different cells and allows a physical channel of a cell to be evenly collided with the physical channels of other cells.
The method of averaging inter-cell interference is disclosed in U.S. Pat. No. 6,473,418 which allows a frequency reuse factor to be near 1 without a complicated cell planning through frequency hopping. However, it is still necessary to consider a user at a cell boundary where the inter-cell interference is dominant to achieve the frequency reuse factor of one.
In the downlink, base station transmission signals of neighboring cells cause interference to the user, and frequency hopping patterns among neighboring cells are different and the transmission power of data channels is differently assigned to users according to locations of the users and their channel status, and therefore, the interference signal pattern are varied each time.
Accordingly, the interference becomes highly non-uniform when a portion of physical channels are active in the neighboring cells or the transmission power of a specific physical channel is higher than that of the other physical channels. That is, a portion of the received symbols suffer from high interference and the interference power and the location of the interfered symbols are unknown.
Error correcting codes are used in order to recover information reliably in partially interfered cases.
Most methods for combating the partial intensive interference with the error correcting codes use Reed-Solomon codes with erasures and errors decoding. Especially, the Reed-Solomon codes are used with an orthogonal signal waveform set in a frequency hopping spread system.
In the methods, an erasure symbol is detected by using the ratio of the outputs of envelope detectors matched to each symbol point of an orthogonal signal waveform set, or by deriving a complicated detecting equation which reduces the decoding error probability of Reed-Solomon codes under the assumption that the interference power is known.
Accordingly, the conventional erasure detection methods are difficult to apply to the systems with other modulation schemes such as M-PSK (M-ary phase shift keying) and other multiple access schemes, or the interference power that the received symbol undergoes is unknown and varied.
Turbo codes and low-density parity checking codes using soft-decision decoding metrics and iterative decoding have drawn a lot of attentions because those codes are designed to provide a better performance than other codes including the Reed-Solomon codes.
For soft-decision decoding of turbo codes and the low-density parity checking codes, the log-likelihood ratios of codeword bits are calculated as the decoding metric inputs, and it is necessary to know the variance of noise added to the received symbol in order to calculate the optimal decoding metric.
However, it is hard to estimate the variance of the noise which each received symbol undergoes in the downlink with fast frequency hopping where the inter-cell interference power in each received symbol is different and unknown. Thus, a receiver conventionally estimates an average value of the noise variance comprising the interference that the received symbols undergo.
When the decoding metric is calculated by using the averaging value of the noise variance comprising the interference even if some of the received symbols are interfered by the strong interference, interfered symbols which are actually less reliable erroneously provide decoding metrics with high reliability, and therefore a channel decoder makes an erroneous decision when performing the iterative decoding operation.